In the oil industry power tongs are used to either grasp and hold pipe, tubing and casing while threaded joints are being made up or broken, or to rotate pipe, tubing and casing in order to make up or break such joints and to effect drilling. In the past, power tongs have been provided with at least one jaw mounted in a displaceable jaw assembly that may be shifted in order to bring the jaw into bearing engagement against pipe. Jaw systems of this design are typically constrained by upper and lower cage plates to which the jaw assembly is connected.
Typically, one or more jaws are caused to advance inwardly by rotating an outer encircling camming surface about the jaw assembly with the pipe located at the center of rotation. This camming surface is typically formed on the inside of a C-shaped ring gear. Camming rollers on the jaw assembly act as a cam follower bearing against this camming surface, and causing the jaw assembly to advance inwardly in response to the inward urging developed by the cam surface as it rotates past the camming rollers. The jaws in many typical designs pivot about a pivot axis that is fixed in relationship to the cage plates, the path followed by the jaws as they are displaced inwardly being arcuate. In other jaw designs the jaws advance radially towards the center of the tong in a sliding action.
Once the jaws have contacted the pipe, a very high level of radial force be applied to the pipe in order to ensure a non-slipping, frictional engagement persists while torque is applied to the pipe. High torque forces are required in order to ensure that the joints in the drill pipe are properly made up, and to turn the bit mounted at the bottom of the drill pipe string where the boring of the earth is occurring.
In order to torque pipe with minimal damage to the pipe surfaces very high radial forces are required when the jaws are equipped with smooth pipe-engagement surfaces. Smooth-faced gripping jaws are particularly employed with casing and tubing that are made of exotic alloys to reduce corrosion. Such high radial forces are achieved by providing a camming surface with a very gradual inwardly-directed slope along which the camming roller is required to advance. Consequently, in order to maximize the radial forces that can be applied to the pipe, it is important that the camming roller should be able to advance along the camming surface with a minimum level of frictional resistance.
An example of a prior art patent addressing this issue is U.S. Pat. No. 5,819,605 to Buck for “Low Friction Power Tong Jaw Assembly”. In this patent it is proposed to provide a friction reducing surface between a jaw roller and the surface of a roller retaining means that secures the jaw roller to an aperture in the body of the jaw assembly. Examples given include pin, ball or sleeve bearings used to support the jaw roller. However, in every case depicted, only a single roller is in contact with the camming surface. Contact occurs along a single line contact that is limited to the length of a single roller.
It would be advantageous to provide a power tong wherein the frictional resistance encountered between the jaw holder and the camming surface is reduced to a low-level. The present invention addresses that objective.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.